ThunderCactus is absolutely 100% right on this. A heavier BB will always travel farther than a lighter BB at the same joule level provided it has enough backspin, even at rather low FPS. The heavier BB starts slower but keeps its speed better over time. What happens is that the heavier BB will eventually catch up with and pass the lighter BB, and since they will both stay in the air about the same time if they each receive optimal hop, the heavier BB will always go farther.